Supernatural - Season 4, Episode 19, Jump the Shark
Jump the Shark
Sam and Dean are contacted by one Adam Milligan who claims to be John Winchester's son. The boys immediately suspect a demon trap and go to investigate. Adam turns out to legitimately be John's son and Dean is furious that John kept Adam's existence from them as well as sheltered Adam from the hunting life. Meanwhile other supernatural forces are at work, which lead to a tragic twist that neither Sam, Dean, or their new sibling could have guessed.